1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine for increasing the air-fuel ratio of an air-fuel mixture in a cylinder in an appropriate range, during transient operation of the engine.
2. Description of the Related Art
Japanese Patent Application Publication No. 5-321726 (JP-A-5-321726) describes a control apparatus that determines the torque fluctuation amount of an internal combustion engine, and controls, through feedback, an air-fuel ratio of an air-fuel mixture formed in an engine cylinder to an appropriate value (lean air-fuel ratio) that is higher than the stoichiometric air-fuel ratio so that the torque fluctuation amount is equal to a target torque fluctuation amount. The control apparatus determines an intake air amount, sets a fuel injection amount based on the determined intake air amount, and changes the air-fuel ratio to a lean air-fuel ratio by changing the set fuel injection amount. More specifically, the apparatus sets a target throttle valve opening amount based on an accelerator operation amount, and makes an actual throttle valve opening amount equal to the target throttle valve opening amount. In addition, the control apparatus executes a control (hereinafter, may be referred to as “lean limit control”) that changes the air-fuel ratio to the leanest possible air-fuel ratio, by decreasing the fuel injection amount while monitoring the torque fluctuation amount.
However, because the above-described control apparatus changes the air-fuel ratio to a lean air-fuel ratio by decreasing the fuel injection amount, an amount of fuel contributing to combustion is decreased when the air-fuel ratio is changed to a leaner air-fuel ratio, even if the accelerator operation amount is constant. As a result, a driver may feel discomfort due to a relatively large decrease in the torque generated by the engine. In addition, the control apparatus changes the air-fuel ratio to a lean air-fuel ratio by decreasing the fuel injection amount set based on the determined intake air amount. Accordingly, under certain conditions, the intake air amount cannot be accurately determined during transient operation of the engine, the air-fuel ratio is excessively lean and therefore, a misfire occurs, or combustion is unstable and the torque greatly fluctuates although a misfire does not occur. Thus, the control apparatus executes the lean limit control, only when an engine load is relatively low, and the engine is in a steady operating state. As a result, the engine operating state, in which the air-fuel ratio can be set to a lean air-fuel ratio, is limited. Therefore, it is difficult to further improve fuel economy.